American Mineralogist, Volume 66, pages 596-600, I98l
Quartz with rhombohedralcleavagefrom Madagascar
Orro W. FLORKE,HnrNz G. Mmrre, JtlncnN Wnrcnr,nt
Institutfiir Mineralogie,Ruhr-UniversitritBochum,Postfach,D-4630Bochum,Germany
AND HOLGER KULKE
DEMINEX, DorotheenstraBe
1, D-4300Essen,Germany
Abstrsct
Anhedral quartz slabsfrom a vein depositin Madagascarshowingsmoothcleavageplanes
parallel to positive rhombohedra r {l0ll) are described.Only two of the three equivalent
pairs ofthe form {l0Tl} show cleavage,although ofdiflerent quality (smoothness).The best
developedplaneslie parallel to Brazil-twin lamellae;the lesswell developedplanesdo not.
We presumesmall tectonically induced thermal shocksto be the releasingmechanism for
rhombohedral cleavagefractures. In combination with hydrolytic weakening and directed
stress,large smoothcleavageplaneswere formed. Sincethe r-facesof quartz are F-faces,they
have a minimum of free surfaceenergy.We assumethis to be the reasonfor cleavageparallel
to positive rhombohedralfaces.
Introduction
Many authors describe qtrartz that, beyond its
well-known conchoidalfracturing, under specialconditions shows cleavage of decreasing quality
(smoothnessor perfection)on r {l0Tl}, z {0lTl},
and m {10T0}, respectively.In most caseshowever
no distinction has been made betweenr and z. The
cleavageparallel to ru usually is not well developed.
Rose (1844) mentioned that cleavage of quartz
may occur to an equal extent parallel to the positive
and negativerhombohedra.The l9th century literature on quartz cleavage was reviewed by Hintze
(1915), and the literature up to 1960 by Frondel
(1962). All authors reported that pronounced platy
cleavage or parting occasionally occurs in vein
qtarlz, mostly in the inner parts of vein fillings.
Rogers (1934) mentioned huge anhedral blocks of
vein quartz from California pegmatites.Sinceparting
wasnot observedparallel to all three equivalentpairs
of rhombohedral faces, Rogers rejected the term
cleavageand concludedthat parting might be due to
polysyntheticDauphin6 1ryinning, mechanicallyproduced during the high-low transformation of the
qvafiz.
Murdoch and Webb (1938) describedquartz with
nearly perfect platy parting in pegmatitic veins from
another California localitv. The slabs showed flat
0003-004x/ 8| /0506-0s96$02.00
surfaceswith pearly luster over areasof l5 to 20 cm'z.
Faint, unevenlyspaced,parallel striationsindicated a
secondand a third direction of parting which were
not as well developedas the first. Thin sectionscut
normal to the principal parting plane showedparallel
lamellae of varying width. The alternating arrangement of these lamellae resembled albite twinning.
Therefore the authors concluded that this peculiar
parting might be explained as a pseudomorph of
single-crystalquartz after triclinic feldspar,though it
was admitted that pseudomorphicreplacement by
single crystalsinstead of fine-grainedpolycrystalline
material is very unusual. Jahns (1953) stated that
there existsno evidencethat large crystalsin pegmatites grow by replacementof other minerals.
The production of cleavagein quartz through thermal shockby Kenngott (18M49) and under directed
stressby Judd (1892) was mentioned by Hintze
(1915).Borg and Maxwell (1956) subjectedqluartz
sand to a directed stressof about 2 kbar at 300'C.
under a confining pressureof about I kbar transmitted by alkaline hydrous solutions.They observed
a remarkableincreaseof fracturing parallel to r and
z, but none parallel to lrt. The percentageof grains
with deformationlamellaedecreasedsimultaneously.
The fractures occurred rarely in parallel setsand occasionallyconformedto more than one crystallographic plane. Bloss(1957)crushedquartz crystalsboth
596
FLORKE ET AL.: QUARTZ WITH RHOMBOHEDRAL CLEAVAGE
at room temperature and between 650 and 700"C. tions perpendicularto the two recognizablecleavage
He obtained a pronounced cleavageparallel to the planesweremeasuredto be 87!2".
rhombohedra r and z and to a much lesser extent
A systemofshort and lesslinsa1,healedfissuresM
parallel to the prism m.
also exists (Fig. 2). Angles of intersection between
theseflssuresand R, or R,, are about 130to l40o in
Geologicalsetting and mineralogy
the same sectionsas above. The M-fissures.which
One of us (HK) collected anhedral fragments of have healed after fracturing, representcharacteristic
large single crystalswith almost perfect platy cleav- accumulations of aligned fluid inclusions. The Rage from qvartz veins in NE Madagascar.The hills, fracturesare unhealed.
Universal-stagemeasurementsproved the R-fracbordering D'Antongil Baie (Fig. l) are part of an old
tures
to be the tracesof rhombohedralfacesand the
cratonic block of Gondwana. Granites, migmatites
M-fissures
to be atgned almost parallel to prism
and gneisseswere cut by a great number of quartz
faces.
veins and basalticdikes during the Late Cretaceous.
At that time the separationof India from MadagasExperimentalresults
car causedintenseshearingand mylonitization of the
X-ray measurements,accordingto the orientation
basementrocks (Hottin, 1969).
Deep lateritic weatheringobscuresthe attitude and method of Keppler (1975),proved that the observed
thicknessof the quartz veins. Furthermore, frequent cleavageplanesare rhombohedralfaces.It is difficult
landslides have produced accumulations of quartz to distinguish between r- and z-rhombohedrausing
blocks at the baseof the slopescontaining the origi- X-ray methods.Although the intensitiesof the reflections l0Tl and 0lTl are different (International Tanal outcrop.
Our samplescome from quartz veins of about 2 m blesfor X-ray Crystalography,1969,Vol. I, p. 465)
thickness, presumably crystallized from hydro- their exact determination is nearly impossible, bethermal solutions at the expenseof the quartz con- cause both reflections are strongly affected by extent ofthe shearedhost rocks.The quartz variesfrom tinction effects.Literature data (International Tables,
milky or slightly smoky to absolutelytransparentin 1962,Yol. III, p. 79; Le Page et al., 1976)are convarious parts of the vein. Cleavageoccursespecially tradictory on the question of which reflection is the
in the inner parts of the vein fi[ing. The remainder more intense.
An unequivocal experimental indication of
exhibits conchoidal fracturing only. The best developed cleavageswere observedin blocks of approxi- whether the rhombohedralcleavageplanesare posimately I m' volume composedof alnost clear quartz tive r- or negative z-planes (Bravais-setof crystalwith only minor trains of inclusions and cloudy lographic axes,seeDana Frondel, 1962)can be obtained by hydrothermal growth. Prisms measuring 5
schlieren(Fig. 2).
x 5 x 15rrm were cut from platy fragmentswith two
natural cleavageplanes forming two faces of these
Sampledescription
prisms. These served as seeds.Growth conditions
Thin sectionsof various thicknessesproved the were: seedtemperature380oC,nutrient temperature
samplesto be single crystals; however, the micro- 400"C, pressure1.5kbar, duration 14 days;nutrient:
structure is characterizedby twin lamellae following broken quartz of the same sample, solvent: I m
the Brazil law. Neighboring lamellae show differ- NarCO, in H, O. After the runs the seedcrystalswere
encesin extinction position of 2o. The lamellaethem- terminatedonly by real facesm, r and z, with r promselves extinguish homogeneously.Specimens with inently developed parallel to the original cleavage
macroscopicallyplanar cleavagesshow systemsof planesof the seed.The facesm and r were morpholong linear fractures parallel to the best developed logically dominant,whereasz-faceswere subordinate
cleavageplanes.We refer to this fracture systemas due to their higher growth rate. The product crystals
R,. The twin lamellae boundaries are frequently exhibited an unusual pseudoorthorhombic-prismatic
identical with the R,-fractures.On the universalstage habit becauseof the prominent developmentof the
a secondsystemof linear fractures,here termed R,r, rhombohedral faces. In agreement with literature
running parallel to a secondpair of rhombohedral values,the measuredvelocitiesof growth VG were:
faces r, could be measured.These fractures are sig- VG- < VG, << YG" (cf. Jost, 1955).Our growth exnificantly shorter than the R, type. The anglesof in- perimentsat different temperatures(pH ll, pressure
tersectionbetweenfracture systemsR, and R,, in sec- 1.5to 3 kbar) show that in the temperaturerange up
FLORKE ET AL.: QUARTZ WITH RHOMBOHEDRAL CLEAVAGE
D
E
gronrtes
t
vetn
w t t hc
1O81m
,
@
o
o
O
t-t-t
lO
2okm
Fig. 1. Sketch of deposit,
to 300oCthe free surfaceenergydifferencebetweenr
and z is so large that z showsmuch higher growth
rates than r. With increasingtemperature,however,
the growth rates of r and z approacheach other and
at 500"C thereis almostno differencebetweenthem.
Twinning was analyzed by etching cuts perpendicularto the c-axis(45Vo}JF,20"C, 30 min), revealing polysyntheticBrazil twinrningparallel to only
that rhombohedral face r which representsthe best
developedcleavageplane.
Untwinned, single crystal slabs(3-5 cm diameter)
of transparent quartz fron diferent localities (Arkansas,Goiaz, Madagascar)were slowly heated(1"/
min) in air (l atm) to l00oC and subsequently
quenchedin water to room temperature.After this
procedure a few linear fractures parallel to the
'K'
r{ \ '.1''-"#
1:
1,
Rrt)*:.-o-',
if
*, .tI
*,
Fig. 2. Photomicrograph of linear rhombohedral cleavage fracture systems R1 and R1y and of healed fissures M; thin section cut
perpendicular to the two macroscopic cleavage systems.
FLORKE ET AL.: QUART:Z WITH RHOMBOHEDRAL CLEAVAGE
rhombohedra were observed.When the maximum
temperaturereached is increased,the fractures becamemore frequent but lesslinear. The reverseprocedure-shock heating and slow seeling to room
temperature-resulted in the sameobservations.
Completely different fractures were produced by
shock heating to temperaturesabout the low-hightransformation of quartz. In this case the slabs
cracked into a fabric of hexagonal, contiguous
prisms, elongatedparallel to the c-axis. Even under
hydrothermal conditions, as well as comparatively
low ratesof heating and cooling the quartz shattered
during the low-high-transformation.
Discussionand conclusions
The quartz cleavageparallel to the positive rhombohedron is in all casesobservedby us to be of different quality on the three equivalent pairs of the
form {l0Tl}. One pair always showsthe best development of cleavageplanesand fractures(R). A second pair is of poorer cleavagequality (R,,). The third
possiblesystemRr' was never observedin our samples. At standardtemperatureand pressure(25"C, I
atm), and in the absenceof water, suitably oriented
stressescaused samples with R-fractures to disintegrate into thin flakes parallel to these fracture
planes,but they do not show any tendencyto form
new ones.
Under the as yet unknown natural conditions,the
huge vein quartz crystalsunderwent an in situ cleaving processwithout the formation of deformation
(Boehm) lamellae or deformation bands. The cleavagedescribedhere must be correlatedwith the structure of quartz.It requiresbreaking of the strong SiO-bonds in the tetrahedra-framework.Fairbairn
(1939)calculatedthe number of Si-O-bonds per face
unit and showedr and z to have a minimum, correspondingwith a minimum of coherenceof the corresponding lattice planes. Though he did not quantitatively discriminatebetweenr and z, he assumeda
decreaseofcoherence in the sequence:
o : {1120},c:
{0001}, ffi, z, r.
Fl6rke (1955,1959)describedthe positiverhombohedron as the only F-form, accordingto the PBC theory of Hartman and Perdoc (1955). In order to explain its morphologicalprominence,it must have the
lowest free surfaceenergy,1.e.,a minillum of loose
Si-O-bonds after cleavage.According to Hartman
(1959), however, the z- and rz-faces should be Ffaces as well, but these do not show a comparable
tendency to cleave. Hartman's method, though excellent,is a qualitative evaluationofsurface energies.
Especiallyin complicatedstructureswith a complex
bonding character,the results derived from it must
be viewed with caution.
The review of literature and our own experiments
show that experimentally produced cleavage in
qtartz, either by directed stressor by thermal shock
without a hydrothermal confining pressure,is poor
compared to the observednatural cleavage.Therefore we assume,without experimental evidence at
present, that the coupled action of a directional
stress,with the resolvedshear stressRSS exceeding
the cleavagestresso" l0Tl, and a confining hydrothermal pressurehigh enough to initiate hydrolytic
weakening (cf. Griggs, 1967) are necessaryto produce cleavagein quartz.
We presume small tectonically induced thermal
shocks(cf. Gay et al.,1978) under hydrothermal conditions in comparativelysmall volumes of the veinbearing rocks to be the releasing mechanism for
cleavagein giant quartz crystals of suitable purity
and suitable orientation relative to the stressdirections. From our experimental results we assumea
rise of temperatureon the order of about l00oC to be
necessary.We considerthe temperaturenot to have
exceeded300"C. Due to the decreasingsurface energy differencesof r and z above300'C the tendency
for cleavageshould not show significant differences.
At higher temperatures,and especially above the
low-high transformation temperature, we expect
from our experimentalresultsthat evenunder hydrothermal conditions quafiz should shatterrather than
cleave.
The cleavagestrengtho" of the positive rhombohedral facesr at low temperatures(far below the lowhigh-transformation)is, from our undershlding of
the quartz structure,lessthan it is for any other form
and especially for the negative rhombohedron z.
Thus the explosivetransformationof the storedstress
energyinto surfaceenergyservesto form large, even
smoothcleavageplanes.Additional hydrolytic weakening moreoverspecificallylowers the ideal and real
cleavageenergyfor r by Si-O-bond breakingand activation of dislocationsin the structural framework.
The proposedmechanismof cleavageaccountsfor
both fracture and cleavagesystemsR' and Rrr, but it
does not explain the di-fferencein cleavagequatty
and the absenceof the symmetricallyequivalent system R,,,. We assumethis to be due to the polysynthetic Brazil twinning parallel to the best rhombohedralplane of cleavage.Brazil twinning along the
600
FLORKE ET AL.: QUARTZ WITH RHOMBOHEDRAL CLEAVAGE
systemR,, doesnot occur. Twin boundariesare twodimensional areas of structural weakness.This
causescleavageto be more easily achievedand explains the better cleavageof R, compared to R,,.
Therefore we assumethe polysynthetic Brazil-twinning to have been formed before cleavage. Polysynthetic Brazil-twinning has been frequently observedin vein quartz (Frondel, 1962,p.87, Figs. 62,
63).
The absenceof the third cleavagesystem R,,, is
due to the orientation of the qvartz with regardto the
directed stress.According to Schmid'slaw the shear
stressr reachesa maximum at r-o* : o/2, if the angle
betweenstresso and gliding plane happensto be 45o.
The interfacial angle between two symmetrically
equivalent rhombohedron faces r is 87". If the directed stresso. bisectsthe interfacial angle between
two equivalent rhombohedron faces r, and r, the
shear stressr nearly reachesa maximum for two
planes at the sametime. The analogousorientation
with regard to three equivalent rhombohedral faces
rr, r, and rr, is, accordingto Schmid'slaw, difficult to
realize.For only ong single orientation will the directedstressbe inclined at 45" to all three faces.The
latter caseis lessprobable. Thus usually two rhombohedral cleavagesystemsare formed.
Our understandingof rupture suggeststhat the distinction betweenfracture, parting and cleavagesuggestedby Fairbairn (1939) cannot be upheld. Even
the conchoidalfracture of quartz is controlled by its
structure (Hoffer, 196l). The term parting should be
preserved for fractures controlled by segregation,
zoning of impurities or exsolution (Buckley, 1934;
White, 1979),whereascleavageshouldbe usedfor all
planar fracturesof crystalswhich are intrinsically or
primarily controlled by the structure.
tobalit. Berichte der D€utschenKeramischenGesellschaft,32,
369-381.
bei der paraFl6rke, O. W. (1959) Regelungserscheinungen
morphen Umwandlung von SiO2-Kristallen. Zeitschrift fiir
Kristallographie, ll2, 126-135.
Frondel, C. (1945)Secondarydauphindtwinning in quartz. American Mineralogist,30, 47-460.
Frondel, C. (1962)The systemof mineralogy.7. Ed. Vol. IIL Silica minerals,Wiley, New York, p. 104.
Gay, N. C., Comins,N. R. and Simpson,C. (1978)The composition of spherules and shatter cone surfacesfrom the Vredefort
structure,SouthAfrica. Earth and PlanetaryScienceLetters,41,
372-380.
Griggs, D. (1967)Hydrolytic weakeningof quanz and other silicates.GeophysicalJournal of Royal Astronomical Society, 14,
l9-3 l.
Hartman, P. (1959)La morphologiestructuraledu quartz. Bulletin
Soci€t€ Frangaisede Mineralogie et de Cristallographie, 82,
33s-340.
Hartman, P. and Perdok, W. G. (1955)On the relations between
structureand morphology of crystals,L Acta Crystallographica,
8.49-s2.
Hintze, C. (1915) Handbuch der Mineralogie, 1. Band, 2. Abt.,
Veith & Co., Leipzig.
Hoffer, A. (1961) Low quartz, on the geometry of its structure
framework in terms of the directed bond. Zeitschrift ftir Kristallographie,I 16,83-100.
Hottin, G. (1969)Les terrainscristallinedu Centreet du Nord-Est
de Madagascar.Documentation du Bureau des G6ologiques,
No. 178,Servaisg6ologiques,Tananarive.
InternationalTablesfor X-ray Crystallography,Vol. I (1962)Kynoch Press,Birmingham, England.
International Tables for X-ray Crystallography,Vol. III (1969)
Kynoch Press,Birmingham, Engiand.
Jahns,R. H. (1935)The genesisof pegmatites.I. Occurrenceand
origin of giant crystals.American Mineralogist, 38, 563-598.
Jost, J. M. (1955) lst Quaterly ProgressReport, Army Signal
Corps, Contract DA-3G039-sc-64689.
Keppler, U., Meier, M., Neifeind, A. and Rohner, C. (1975)X-ray
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Murdoch, J. and Webb, R. W. (1938) Notes on some minerals
from southernCalifornia. American Mineralogist, 23, 349-355,
p.354.
Nicolas,A. and Poirier, J. P. (1976)Crystallineplasticity and solid
Acknowledgments
stateflow in metamorphicrocks.Wiley, New York, p. 200.
We gratefully acknowledge the thorough revision of the manuLe Page, Y. and Donnay, G. (1976) Refinement of the crystal
script by Walter V. Maresch Ph.D., and we would like to express
structureof low quartz. Acta Crystallographica,32,2456-59.
our most sincere thanks to Mrs. Helene Nowara and Mr. Dieter
Rogers,A. F. (1934) Unique occurrenceof vein quartz in MariDettmar for their valuable technical assistance.
posa Country, California. Proceedingsof the Geological Society
of America, 1934,327-328.
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Manuscript received, fune 16, 1980;
Fl6rke, O. W. (1955) Strukturanomalienbei Tridymit und Crisaccepted for publication, January 5, I98I